Susan Siderman (left) runs with the Mercury Masters, a group of fit and fab women over 50. (Rahav ‘Iggy’ Segev / Photopass.c)

Some 15,000 scenesters will crowd into Prospect Park today for a sold-out event, but the hot ticket isn’t a Bon Iver concert at the bandshell or a “Twin Peaks” marathon on a giant outdoor screen. It’s the chance to run 13.1 miles at 7 o’clock on a Saturday morning.

After last year’s Brooklyn Half-Marathon sold out in just a couple of days, the New York Road Runners changed the course for this year’s race to accommodate almost three times as many runners. More than 17,000 are registered, with 15,000 expected to finish, compared to fewer than 6,000 last year.

This time around, spots filled up in just 10 hours, with runners fighting for the chance to pay upward of $45 to run along Ocean Parkway on a likely too-warm day.

“The half-marathon has become the distance to do,” explains Ryan Lamppa, the media director for Running USA, a nonprofit that works to promote the sport. Since 2003, the half-marathon has been the fastest-growing road-race distance in the country, with double-digit growth every year for the past six years.

“It’s become something like a marathon-lite; it’s not as long, but it’s as satisfying,” he says, noting that many runners check a full marathon off their bucket list and then focus on half-marathons because they’re easier on the body but still feel like a big accomplishment.

These days, it seems that everyone from feisty grandmas to independent-minded high-school freshmen are running one.

Barbara Rothenberg, 66, is a grandmother of six who lives in Brooklyn and is looking to finish her second half-marathon today.

“It’s not as punishing as the full marathon,” says Rothenberg, though she’s also aiming to run the NYC Marathon for the first time this year. The fourth-degree black belt picked up running at age 62, and so far has been relatively injury-free.

“I’m going to do this running until my body starts hurting,” she says.

Another grandma on the run, Susan Siderman, 67, took up the sport in 1980 as a way to get some quick exercise between shuttling her two daughters around Long Island. In the decades since, she’s completed 45 marathons and countless (“I don’t even know how many”) half-marathons.

Women have been key to the half-marathon’s rise in popularity, accounting for 59 percent of the race finishers in the US last year. Eight years ago, it was a fairly even split between the sexes.

A retired teacher living in the Financial District, Siderman is one of the original members of the Mercury Masters, a NYRR running team for women over 50. At a recent 10-kilometer race in Central Park, she and her sassy teammates donned purple jerseys to run the race and cheer each other on — all while ogling the occasional younger fellow trotting by.

While Siderman has slowed down since her best half-marathon time of an hour and 48 minutes, she doesn’t mind. Running with the Mercury Masters, a k a the “MeMas,” is about the “five F’s,” she says: “Female, fifty, fitness, fun and friendship.”

On the other end of the age spectrum, 15-year-old Lauren Mantel is running her first half-marathon in Brooklyn today in support of the HOPE program, a nonprofit that helps those in poverty. A freshman at Columbia Prep, she took up running on her own when she was looking for an alternative to the regimented schedule of the varsity swim team.

“I just decided school sports weren’t for me,” she says, recalling how her friends and family laughed when she told them of her plans to take up distance running on her own. Now, she says, “Here I am . . . I’m going to do a half-marathon, and I’ve proved them all wrong.”

Part of the sport’s appeal, says Mantel, is the chance to explore new parts of the city — the Upper West Sider says she’d never been to Brooklyn before doing a 6-kilometer race in Prospect Park in December. “[It’s] given me the opportunity to see places in the city I wouldn’t normally see,” she says.

Today’s Brooklyn run has the unique appeal of ending on the Coney Island boardwalk. But runners are using half-marathons as an excuse to travel to locales even further afield than the outer boroughs.

Every year for the past decade, approximately 30 new half-marathons have sprung up across the country. According to Lamppa, cities and towns recognize their powerful ability to bring in tourism dollars, and race organizers recognize a booming business opportunity, creating tailor-made races — including raucous runs with a live band every mile.

Robert Sjoberg, 27, a data analyst for a software company who lives on the Lower East Side, recently went to Tennessee to run the Rock ’n’ Roll Half-Marathon and hear some country music, a newfound passion.

He and seven friends from New York made the trek, as did several pals from Seattle and North Carolina, for a weekend of bar hopping, fine dining and, oh yeah, that little jog.

“The race itself was extremely difficult for me,” he says, noting the hilly course and humidity. But, he adds, “the getaway was fantastic.”

For New Yorkers who think they’re too busy to travel or train, Lamppa points to Oprah, who helped kick-start the sport’s ongoing boom in popularity in 1994.

That’s when she ran the Marine Corps Marathon in Washington, DC, in an impressive 4:29:15, proving that the sport isn’t limited to stick-thin men trying to qualify for Boston, and that anyone can find the time to train.

*Niketown Run Club: The free, fun running club accommodates all levels; meets every Tuesday and Thursday at 6:30 p.m., and Saturdays at 9 a.m. 6 E. 57th St.; 212-891-6453

*New York Road Runners: The city’s pre-eminent running organization offers classes for everyone, meeting once or twice a week for 10 weeks and costing $105 or $180 for NYRR members, and $125 or $250 for nonmembers; nyrr.org/get-training

*Team in Training: Join the NYC chapter of the world’s largest endurance sports-training program and get free coaching for your big race in exchange for fund-raising for the Leukemia & Lymphoma Society; teamintraining.org/nyc/

Training tips

“Unlike a marathon, you don’t need to sacrifice your social life or other activities to train [for a half-marathon]” says Jonathan Cane, a JackRabbit Sports running and triathlon coach who preps first-timers by building them up to running four times a week, 25 miles total. Here, he shares some tips: